The invention relates to an energy converter for supplying electric energy from an energy source to a load, the energy converter comprising a transformer having a primary side and a secondary side, the secondary side being adapted to be connected, in operation, to the load, at least a first and a second series-arranged, controllable switch to be connected, in operation, to the energy source, diodes arranged anti-parallel to the first and the second switch, and comprising a control device for generating control signals with which the first and the second switch are opened and closed for generating an alternating current in the primary side of the transformer, the control device comprising means for comparing a threshold value with the value of a quantity which is related or equal to a change of the voltage per unit of time at a node of the first switch and the second switch for determining switching instants of the first and the second switch.
An energy converter of this type is known per se from, inter alia, U.S. Pat. No. 5,075,599 and U.S. Pat. No. 5,696,431. In this converter, the load is often a rectifier and the energy source is a DC voltage source. Together with the load, the energy converter has for its object to convert a DC input voltage of the energy source into a DC output voltage of the load. However, the load may also comprise a different device than the rectifier, which device is fed with an alternating voltage. The energy converter may thus consist, inter alia, of a DC/DC converter and a DC/AC converter.
For a satisfactory operation of the energy converter, it is important that the switches for generating the alternating current are switched on and off at the right instant. The frequency at which the switches are switched on and off defines the mode of operation of the converter. If the frequency is sufficiently high, the energy converter operates in a regular inductive mode. In this mode, the phase of the current through the primary side of the transformer trails the phase of the voltage at the node. After a current-conducting switch is opened, and after the diode of the other switch has started to conduct the current, the other switch can be opened. In that case, there are no switching losses. The time interval in which both switches are opened is referred to as the non-overlap time.
The converter operates in the near-capacitive mode when the switching frequency of the switches, and hence the frequency of the alternating current through the primary side of the transformer is decreased to a point where the alternating current is at least almost in phase with the alternating current at the node. After the current conducting switch is opened and before the diode, which is arranged anti-parallel to the other switch, starts to conduct, the direction of the current through the primary side of the transformer is reversed. Hard-switching takes place if the other switch is closed in that case. This means that switching takes place at an instant when there is a voltage difference across the relevant switch. This will result in switching losses.
The converter operates in the capacitive mode when the frequency at which the switches are switched is further decreased to a point where the alternating current through the primary side of the transformer is in phase with, or even leads the phase of the voltage at the node. The switching losses also occur in this mode.
Generally, it is desirable that the energy converter operates in the inductive mode. To this end, it is important that the non-overlap time is chosen to be sufficiently long to prevent hard-switching, i.e. switching losses. However, the non-overlap time is bound to a maximum because hard-switching also occurs in the case of a too long overlap time so that switching losses occur.
To determine the overlap time for an energy converter operating in the inductive mode, it is known to provide the control device with means for comparing the value of a quantity which relates or is equal to the value of a change of the voltage per unit of time at a node of the first and the second switch, on the one hand, with a threshold value, on the other hand, for determining the switching instants of the first and the second switch. More particularly, the instant when the other switch must be closed, is determined by measuring the current flowing through a capacitance of the energy converter, which capacitance is incorporated in the energy converter in such a way that it reduces the value of the change of the voltage at the node per unit of time. The other switch is closed at the instant when the value of this current decreases and becomes equal to a relatively small positive threshold value. In accordance with a practical elaboration, the switching instant is determined by comparing the voltage across the current-sense resistor with a reference voltage by means of a comparator. This sense resistor may be arranged in series with said capacitance, or it may be incorporated in the alternating current path via a capacitive current divider. A drawback of the known energy converter is that the comparator, which is operative on the basis of relatively small input signals and relatively small slopes, can react in a delayed manner. As a result, the relevant switches may be switched on too late. This in turn may mean that hard-switching as yet occurs in the inductive mode, resulting in switching losses.
It is an object of the invention to provide a solution to the above-mentioned problem. It is also an object of the invention to provide an energy converter which, when operative in the near-capacitive mode, can reduce the switching losses to a minimum.
According to the invention, the energy converter is characterized in that the control device is adapted to determine a reached maximum value of said quantity and to determine the threshold value on the basis of the determined maximum value of the quantity. Since the threshold voltage is determined on the basis of a determined maximum value of said quantity, it is possible to compensate for said delay and for a comparator possibly used in the control device.
The threshold value can be particularly chosen on the basis of the maximum value in such a way that, when the energy converter operates in the near-capacitive mode, the relevant switch is closed when the alternating voltage at the node has reached an extreme value. This extreme value results in the switching losses being minimized. The reason is that the voltage difference across the switch which is closed at that instant is minimal.
Particularly, the threshold voltage is equal to a factor K times the maximum value, in which K has a value of between 0 and 1. This factor K may be particularly chosen to be such that the switching losses are minimal in the near-capacitive mode. In the inductive mode, it then holds that the overlap time has such a non-critical value that there will be no switching losses at all.
It is therefore preferable that the factor K is determined in such a way that one of the switching instants coincides with the instant when the voltage at the node assumes an extreme value when the frequency of the alternating current through the primary side of the transformer is so low that this alternating current is at least substantially in phase with the voltage at the node. The factor K is thus determined in such a way that the switching losses are minimal in the near-capacitive mode.
The energy converter preferably also comprises at least a capacitance for limiting the value of a change of the voltage at the node per unit of time, the value of said quantity relating to the value of the current through the capacitance. If this capacitance is not present, the voltage at the node per unit of time will have a very large change and will be dependent on parasitic capacitances. If the semiconductor switches do not have large parasitic capacitances, it is therefore advantageous to include the capacitance for limiting the value of the change of the voltage at the node per unit of time. This will often be the case in practice.
In the latter case, it particularly holds that the factor K is determined in such a way that one of the switching instants coincides with the instant when the current through the capacitance becomes zero, while the value of the current preceding said instant decreases to zero when the frequency of the alternating current through the primary side of the transformer is so low that this alternating current is at least substantially in phase with the voltage at the node. The control device may then comprise a current peak detector which is connected via a first measuring capacitance to the node for determining said maximum value.
In accordance with a further elaboration of this variant, the control device further comprises a multiplier which is connected to an output of the peak detector for multiplying the maximum value by the factor K, a second measuring capacitance and a comparator which is connected to an output of the multiplier and is connected to the node via the second measuring capacitance, the comparator being adapted to determine the instant when an output signal of the peak detector is equal to an output signal of the comparator.
Generally, the first and the second capacitance are formed by at least one and the same capacitance in this case.
These and other aspects are apparent from and will be elucidated with reference to the embodiments described hereinafter.